1. Field of the Invention
The present invention relates to an apparatus and method for decoding a moving picture, and more specifically, to an apparatus and method for decoding a moving picture decoding a video stream signal compression-encoded in a digital signal, as in the MPEG2 (Moving Picture Experts Group) method or the like.
2. Description of the Background Art
Digital broadcasting has been introduced as substitution for the traditional analog broadcasting for the purpose of realization of high picture quality, multichannel display and high functionality, and improvement on other characteristics. In the digital broad casting, the MPEG2 method has been adopted as a compression encoding method for a digitized moving picture.
FIG. 12 is a representation for describing a configuration of an MPEG2 video stream.
Referring to FIG. 12, a MPEG2 video stream signal is an aggregate of GOPs (Groups of Pictures) and each GOP is an aggregate of pictures.
Included in a GOP are three kinds of pictures: I picture, P picture and B picture. I picture is a picture obtained by coding of information in a frame only, which is generated without application of inter-frame prediction and at least one I picture is contained in a GOP. P picture includes both information on intra-frame coding and forward inter-frame predictive coding which performs prediction based on a previous, reproduced picture. B picture is a picture formed by bidirectional prediction. Decoding of P and B pictures require inter-frame prediction, so a processing amount in a MPEG decoder is large, in turn requiring much of a memory capacity.
A picture is divided into slices, which are units, a slice is an aggregate of some number of macro blocks and according to stipulation in the MPEG2 standards, one slice never includes macro blocks in a second row. That is, in a case where one row in a lateral direction is constituted of 40 macro blocks, the 40 macro blocks in the one row can be divided into any number of slices with one macro block as a unit, but a macro block in a second row has to be included in another separate slice without fail.
While individual macro blocks each have no absolute position information, a slice header SH is inserted at a leading position and includes header information for showing what vertical position on a screen each slice is an aggregate of macro blocks included in. That is, a vertical position of macro blocks following the slice header SH can be known based thereon.
Information called a sequence header is added prior to each GOP. Included in the sequence header is information on a picture, such as the number of pixels in a horizontal direction of the picture, the number of lines in a vertical direction of the picture, a frame rate and so on.
FIGS. 13 to 16 are representations for describing format examples of the MPEG2 standards. For example, ARIB (Association of Radio Industries and Businesses) stipulates that in the BS digital broadcasting, programs be prepared using one of formats including 1080i, 720p, 480p and 480i. Accordingly, a moving picture decoding apparatus receiving and reproducing broadcast can desirably reproduce any of the formats. Note that the figures in each of the formats show the number of effective scanning lines and an alphabetical symbol i shows the word of interlace, that is an interlaced scanning, p the word of progressive, that is a progressive scanning.
Referring to FIG. 13, the 1080i format is used for displaying 30 frames per sec in an interlaced scanning on a screen of 1080 dots in height ×1920 dots in width.
Referring to FIG. 14, the 720p format is used for displaying 60 frames per sec in a progressive operation on a screen of 720 dots in height×1280 dots in width.
Referring to FIG. 15, the 480p format is used for displaying 60 frames per sec in a progressive operation on a screen of 480 dots in height×720 dots in width.
Referring to FIG. 16, the 480i format is used for displaying 30 frames per sec in an interlaced scanning on a screen of 480 dots in height×720 dots in width.
A process decoding an MPEG2 video stream signal from a compressed state to an original picture data can be performed on one MPEG decoder LSI, for example.
However, in order to add a further value to a digital TV receiver, a desire arises for a multichannel display mode displaying plural pictures simultaneously.
FIG. 17 is a table for describing an example reproduction mode in a multichannel case.
Referring to FIG. 17, in a case of an HD (High Definition) normal reproduction mode, displayed in channel 1 is a picture of the 1080i format.
In a case of a SD (Standard Definition) four channel multi-reproduction mode, a screen is split into 4 portions and 480i format is displayed on each screen portion.
In a case of normal HD+ daughter screen SD, a picture of the 1080i format as channel 1 and a daughter screen of the 480i format as channel 2 are displayed together on the screen.
In a case of an HD two screen reproduction mode, pictures of the 1080i format are displayed in respective channels 1 and 2.
In a case of a 480p four channel multi-reproduction mode, a screen is split into 4 portions and pictures each of a 480p format in channels 1 to 4 are displayed on respective 4 picture portions.
In a case of an HD reverse reproduction mode, a screen is split into two portions, a picture of the 1080i format is displayed on a first screen, and a picture of 1080i format reproduced in a reverse order is displayed on a second screen.
In a case of an HD double speed reproduction mode, pictures of the 1080i format are displayed at a double speed.
In a case of an HD three channel reproduction mode, a screen is split into 3 portions and pictures each of the 1080i format are displayed on the respective screen portions.
Typical examples of the reproduction modes are described above. In order to display pictures of various kinds of formats on one display screen, a need arises only for providing decoding circuits performing decoding processing on respective pictures, but in the case, a total circuit scale of a moving picture decoding apparatus is large and the apparatus becomes costly to an unreasonable level. Therefore, it is conceived that picture data of plural channels is subjected to time-division decoding in one decoding circuit.
Referring again to FIGS. 13 to 16, a processing amount of each of the formats for decoding is such that if a processing amount of the 480i format is almost 1, a processing amount of the 480p format is almost 2. A processing amount of the 720p format is almost 4 to 5 and a processing amount of the 1080i format is almost 6. A memory capacity required for a decoding process is such that if a memory capacity in a case of the 480i format is almost 1, a memory capacity for a case of the 480p format is almost 1 and a memory capacity for a case of the 1080i format is almost 6.
FIG. 18 is an illustration for describing a time required for decoding in each of reproduction modes.
Referring to FIG. 18, in a case of the HD normal reproduction mode, decoding of picture data to be displayed in one frame period is performed with some margin. An HD mode and an SD mode are included in the contents of an MPEG broadcast stream and the modes vary according to a broadcast time zone; therefore, it is customary that a processing capability of a decoding circuit for decode processing is set to a capacity with which one screen display of the HD normal reproduction mode can be processed with a margin at some level.
In a case where a decoding circuit with such a capacity is mounted on a moving picture decoding apparatus in the SD four channel multiple reproduction mode, a decoding process ends within one frame period even if data of channels 1 to 4 are subjected to time division decoding; therefore, the reproduction can be performed with no problem.
In the cases of reproduction modes such as of the normal HD+ daughter screen SD, the HD two screen reproduction, the 480p four channel multi-reproduction, the HD reverse reproduction, the HD double speed reproduction and the HD three channel reproduction, it takes a processing time longer than one frame period for performing a decoding process on data to be displayed in one frame. Accordingly, a total of processing amounts exceeds a capability of a decoder, thereby disabling inputted picture data in all of channels to be reproduced. Furthermore, in a case of performing parallel processing as well, processing within one frame period is harder to be realized at a low cost with a small bus width and a small memory capacity.